As is well known, apparatus for MR imaging of a subject includes an MR magnet to generate a magnetic field to be applied to the subject, an RF transmit coil arrangement for generating an RF pulse in a transmit stage to be applied to the subject to be imaged such that the subject generates an MR signal in response to the magnetic field and the RF pulse applied, a receive coil arrangement for acquiring MR signals in a receive stage and a signal processing system. In recent years phased array surface coils have been used to feed separate signals to a plurality of separate channels for receiving the MR signals for carrying out signal processing in the separate channels by which an image is generated.
Fast MR image technology has been the development of sophisticated multi-element phased array coils which are capable of acquiring multiple channels of data in parallel. This ‘parallel imaging’ technique uses unique acquisition schemes that allow for accelerated imaging, by replacing some of the spatial coding originating from the magnetic gradients with the spatial sensitivity of the different coil elements. However, the increased acceleration also reduces the signal-to-noise ratio and can create residual artifacts in the image reconstruction. Many parallel acquisition and reconstruction schemes are known.
It is necessary that for phased array coils to operate, decoupling of the signals of the coils must be obtained so that they can be independently processed.
Typically decoupling is obtained by the well known arrangement in which the coils are partly overlapped by a ratio of overlapped to non-overlapped area which acts to decouple the two coils simply by the geometry without any pre-processing of the signals prior to processing in the separate channels of the processing system.
It is known that stacking of coils one on top of the other, or one inside of the other, will provide a very effective arrangement of the phased array coils since they can be of the same size and are located at the same position relative to the sample.
However it is accepted that such stacked coils provide a very strong mutual inductance which prevents decoupling of the signals.
In a paper entitled “Study on the Decoupling of Stacked Phased Array coils for magnetic resonance imaging” by Dandan Liang et al published in Progress in Electromagnetics Research Symposium Proceedings Suzhou China Sep. 12 to 16, 2011 there is described a pre-processing system which uses a complex algorithm to perform the post-processing for obtaining the decoupled signals of the stacked coils. Even using this complex algorithm, the coils must be spaced one from the other by a distance greater than 0.5 cms and typically as much as 3.0 cms. This of course significantly reduces the desirability of the stacked arrangement since it is highly desirable to have the coils substantially directly overlying or as close as possible to the same plane. The best advantage is obtained when the coils are in the same plane, or surface if they are not planar, and of the same shape in that plane. However the difficulty of decoupling is reduced when the coils are axially spaced. This paper does not provide actual imaging results, and since actual imaging is more complicated that bench top testing because both pre and post processing of the images is required. Artifacts will most likely result from the pre and post processing of the images.
Current phased array coil designs where the individual coil elements are arranged side by side with partial overlap involves a trade-off between the size of the coil elements and the number of individual elements for a certain overall size of the coil. As the ratio of overlapped portion to non-overlapped portion is set in order to provide the required decoupling, the number of coil elements which can be arranged in a required is dependent on the size of the coil elements.
Current design is also limited by the relationship between coil element size to the penetration achieved since axial penetration of the image is proportional to transverse dimension of the coil.
Current design is also limited by the relationship between coil element size and SNR values. Large size elements produce poor SNR values and small size elements produce good SNR values.
Current imaging design involves a trade-off between image penetration and SNR. In order to achieve good penetration, birdcage volume coils are used because of the uniformity of the RF field in the entire volume of the coil, but do not have good SNR values compared to phased array surface coils. Phased array surface coils have good SNR, but do not have good penetration compared to the birdcage volume coil. To achieve good penetration with the phased array coil, the size of each element has to be increased, but at a sacrifice of good SNR.
The higher the numbers of coil elements in the phase encode direction, the higher the acceleration achieved in parallel imaging.
Current imaging design involves a trade-off between in parallel imaging and SNR, the higher accelerate(R), the more SNR decreased and more image artifact generated.
      SNR    PMRI    =            SNR      full              g      ⁢      \      ⁢              R        _            
The stacked array coil can greatly improve the relationship between penetration and SNR and fast imaging.